16 month old hitting

I have been looking after a 16 month old since she was 11 months old. She was so sweet and gentle and gave everyone hugs and then about a month ago she started hitting, pinching, and squeezing cheeks. I try to tell her be gentle, no hitting, etc.
Another baby started about the same time as this behavior started, so I don't know if its jealousy/attention seeking or if it would have happened anyways.

Anyone have experience with this age, do time outs work? Hopefully this is just a phase, as I feel like I can't turn my back on her without her hurting someone. There is a 15 month old and a 13 month old, (as well as 2 older children who she doesn't touch). The other 2 babies are starting to pick up on her behavior.
Help!

I don't find time outs work at that age. I do what I call a "time away"....the child is removed from the others (just to a different area in the playroom) and given a toy or book to read. I tell them in simple words why they are separated "No pinching. Not nice. You play here for a bit". Than after a minute or two I encourage them to come back and "Play nice. No pinching". I will keep doing this over and over. They are still young and we have to teach them what is acceptable play and what isn't.

I find once they get closer to two than timeouts start to work. Some kids can understand time outs around 20 or so months though depending on their mental age.

I have a boy that is 2 1/2 that is having this issue also. He was biting about a year ago and his parents and I managed to get him to stop this. In the last 6 months he has been hitting. I've found that the time outs don't really work. I've been recently shadowing him and this seems to work better. If I'm unable to shadow him because I'm doing something where I can't follow him, I either have him follow me (say to the bathroom and have him wait just outside the door)or if I'm doing something in the kitchen, I'll have him sit at the table. Time will tell if I can teach him to stop being aggressive.

Time out's don't always work for kids either. Find what they love (a toy, an activity, a book etc) and use that. For example, my previous handful was only allowed to sit on a pink chair at afternoon snack if she had a good day. For morning snack and lunch she had to sit on a green chair. I couldn't believe how quickly the "bad" behaviour stopped after I stopped doing time outs and enforced this rule. She would sometimes spend most of her day in time out and the behaviour never changed!!

Sounds like you have the same issue with the colored chairs that I have. Do you find that it's a constant argument who gets which color chair. Here it's the girls arguing over the purple chair or which boy gets the blue chair. If there's too much arguing about a certain chair I remove it for a couple of days.

daycaremom9....No, the children never argue over the chairs. Funny enough, they all have their "spots" (that they just assigned themselves lol) and whatever colour of chair happens to be there is the one they sit on. I didn't realize that they always sit in the same spot until one day I had put their sippy cups out before they got to the table and they all sat in their spot and moved the cups to the right child lol. I have 2 green and 2 pink chairs and they are still young enough (or laid back enough) to not care who gets what colour. They are 18 months, two 2 year old's and a 2.5 year old. The 4 year old I had was the only one who cared what colour of chair she sat in and would run to the table to get a pink one. When I noticed time outs didn't phase her I decided to use the pink chair as a reward and it worked beautifully!